The following patent publications are believed to represent the current state of the art:
U.S. Pat. Nos. 5,814,103; 5,876,442; 5,928,283; 6,007,579; 6,066,171; 5,653,751; 6,596,026; 6,464,725; 5,391,202; 5,384,606; 4,074,368; 4,994,082; 5,628,798; 5,222,981; 4,172,297; 5,769,890; 4,892,543; 4,373,218; 4,968,127; 4,759,761; 4,976,732 and 5,769,889;
Published U.S. Application 2001/018,612;
Published PCT Applications WO 94/07,435; WO 00/38593 and WO 83/01566;
Foreign Patent Publications DE 4,403,326; EP 1,092,402; EP 0,419,740; GB 2,181,355; EP 0,897,702; EP 0,212,616; DE 3,428,895 and DE 19,501,444.